They added weights to a Palm Pilot strapped to test subjects' wrists and asked them to perform a text input task using its keyboard. The task took two minutes and was repeated five times with 30 seconds in between each set. Imagine looking at a watch so that your forearm is parallel with your ribs and you're in the same position.

A weight of 540 grams on the wrist turned out to be enough to make subjects feel significantly tired, and increase the electrical activity of the muscles. The authors say:

"In the short term, these stresses can result in sensations of pain and discomfort. In the long term, it is proposed that prolonged exposure to such stresses may have serious pathomechanical effects, including the possibility of developing musculoskeletal disorders."

I'm not surprised - 540g is like carrying a full can of beer on your wrist the whole time, and not being allowed to drink it. The Palm Pilot alone - at 20g - was found to be safe. Bear that in mind before you strap your laptop to your arm.